Recently, as miniaturization and lightness of portable electronic devices have rapidly progressed, the necessity for miniaturization of a battery and an increase in capacity of the battery used as a driving power supply of the portable electronic devices has increased. Among others, a lithium secondary battery has an operation voltage of 3.6 V or more, which is three times higher than that of a nickel-cadmium battery or a nickel-hydrogen battery mainly used as a power supply of the portable electronic devices, and has high energy density per unit weight, such that the use of the lithium secondary battery has been rapidly extended.
The lithium secondary battery is mainly divided into a cylindrical battery, a square type battery, a pouch type battery, and the like, according to an appearance thereof, and is divided into a lithium ion battery, a lithium ion polymer battery, a lithium polymer battery, and the like, according to a form of an electrolyte solution.
Among them, the demand for the square type battery and the pouch type battery having a thin thickness has increased in accordance with the trend toward miniaturization of a mobile device. Particularly, the interest in the pouch type battery having an easily changed form, a low manufacturing cost, and light weight has increased.
Generally, the pouch type battery means a battery in which an electrode assembly and an electrolyte are sealed in a pouch type case of a laminate sheet configured to include a resin layer and a metal layer. The electrode assembly received in the case has a jelly roll type (winding type) structure or a stack type structure.
Meanwhile, in the pouch type secondary battery, operation potential of the battery is high, such that high energy may instantaneously flow, and cathode materials have chemical activity significantly increased due to overcharge or a short-circuit to rapidly react with an electrolyte, thereby generating a large amount of gas. As a result, the battery explodes due to a rapid increase in pressure or temperature in the battery, such that peripheral devices or a human body may be damaged.
Particularly, this problem may cause a more series large accident in the case of a middle or large-sized battery pack including a plurality of battery cells.
Since the middle or large-sized battery pack includes a plurality of battery cells or unit modules in a frame having a predetermined space, as the plurality of battery cells or unit modules are expanded, pressure in a case may be rapidly increased.
Due to the above-mentioned reason, an attempt to solve a stability problem of the secondary battery module has been continuously conducted. However, in most cases, a method of attaching a separate component or device is used, such that an additional process or equipment is required at the time of producing the battery, thereby decreasing productivity and energy density of the battery and increasing a production cost of the battery.
A battery cell disclosed in Korean Patent Laid-Open Publication No. 2004-0081521 is configured to include a case receiving a jelly-roll type (winding type) electrode assembly.
The battery cell according to the related art further includes a safety plate depressed from an outer side of the case to an inner side thereof.
In order to solve a problem that the battery explodes due to an increase in internal pressure or temperature, the battery cell according to the related art is provided with the safety plate broken at the time of an increase in the pressure to thereby be used as a path through which gas is discharged.
However, in the battery cell according to the related art, when the internal pressure increases, since a sealed portion of the case is first opened and the safety plate is then broken while stress is transferred up to a position at which the safety plate is formed, threshold internal pressure at which the safety plate is broken may not be controlled.
In addition, in the battery cell according to the related art, since the threshold internal pressure at which the safety plate is broken may not be concentrated, the battery cell may explode without break of the safety plate even in a state in which the safety plate does not withstand the internal pressure.
Therefore, the development of a technology capable of controlling pressure at which a case of a battery cell may be securely broken has been demanded.